1. Field of the Invention
The present invention relates to a buffer device adapted for use in an image forming apparatus and allowing displacement of a reciprocating member with a rapid start and a constant speed, and more particularly to a buffer device for use in an apparatus for causing reciprocating motion in a reciprocating member such as an original table or an optical system by means of a rotary torque from a motor or a clutch connected thereto through a belt or wire etc.
2. Description of the Prior Art
FIG. 1 shows a copying apparatus with a reciprocating optical system utilizing a buffer device. At the start of exposure step, an illuminating lamp 4 and a first mirror 5 constituting the movable part of the optical system are driven toward the right, in the illustration, at a speed identical with the peripheral speed of a photosensitive drum 3, and simultaneously a second mirror is displaced also to the right at a speed half of the above-mentioned speed.
The exposing optical system in a copier requires a mechanism for driving said first and second mirrors with an exact speed ratio of 2:1, and a buffer device is employed in driving such an optical system. An example of a mirror drive mechanism utilizing a conventional buffer device is shown in FIG. 2 wherein there is provided, between a stationary pulley 63 and a drive pulley 65, a tension pulley 68 rotatably mounted on a pulley mounting plate 67 swingable around an axis 66, said plate being biased toward the drive pulley 65 by means of a spring 69.
In the illustration the exposure of the original is performed in the direction indicated by the arrow. At the moment when the exposure step is completed and the drive pulley 65 starts to be reversed in a direction opposite to the arrow, the tension side and the slack side of a wire 70, which is used as a belt, are inverted whereby a wire portion 70c is tensioned and causes a counterclockwise of the tension pulley 68 around the axis 66. On the other hand a wire portion 70b becomes slack, and the movement of the drive pulley 65 is not transmitted completely to a first mirror 51 and a second mirror 53 until said slack is absorbed by the spring 69. Such incomplete transmission does not cause, however, a serious problem as imagewise exposure is not performed in the reversing motion in such optical system. On the other hand, at the moment when the drive pulley 65 enters the exposure step, the wire portions 70b and 70c respectively become the tension and slack sides, causing a clockwise swinging of the tension pulley 68 around the axis 66 in order to absorb the slack of said wire portion 70c. In the initial phase of the exposure step the speed of the movable mirror does not coincide with that of the drive pulley because of the facts that the slack absorption by said spring is realized much slower than the slack formation in the wire, and that a vibration is caused in the optical system by the starting shock of the motor or the clutch. The exposure of the original cannot therefore be conducted in said initial phase due to the difference between the mirror speed and the drum peripheral speed. The exposure becomes possible only from a point where the mirror speed becomes identical with the drum peripheral speed, and the portion before reaching said point constitutes an originally unnecessary lead-in portion which will unnecessarily enlarge the dimensions of the apparatus. Also the shocks resulting from switching between the slack state and tensioned state significantly reduces the life of the wire. Furthermore, as the tension of the tension spring is selected smaller than that of the wire, the mirror is apt to be driven, in the reversing motion thereof, with vibration which frequently gives rise to undesirable effects on various parts of the apparatus.